Macroscopic magnetization of primordial plasma by virial shocks

We show that galaxy-cluster virial (i.e., structure-formation accretion) shock observations, in particular of synchrotron emission, imply ≳ 1% magnetization of a macroscopic, ≳ 10¹⁶Debye-length layer downstream, challenging high Alfvén-Mach collisionless-shock modelling. Unlike similar shocks in supernova remnants or relativistic shocks inγ-ray burst afterglows, where macroscopic magnetized layers were attributed to preexisting or non-resonant cosmic-ray streaming-seeded substructure, virial shock upstreams are both weakly magnetized and pristine. Hence, some mechanism must generate macroscopic sub-structure out of the accreted primordial plasma, and may similarly dominate additional high-Mach shock systems.